The present invention relates to an isothermal port and method for a microwave network analyzer.
In a preferred embodiment, the present invention is intended to be utilized with a microwave network analyzer system such as the Hewlett-Packard Company HP 8510C Microwave Network Analyzer family of products, which provides magnitude, phase and group delay measurements from 45 MHz to 110 GHz.
The HP 8510 network analyzer system has the capability to make real-time error-corrected measurements of components and devices in a variety of transmission media. In general, all that is required is a set of known devices (standards) that can be defined physically or electrically and used to provide a reference for the physical interface of the test devices.
Some of the standards used to calibrate (and verify the performance) of microwave network analyzers are coaxial impedance standards (airlines). Beadless airlines have a center conductor, air dielectric, and an outer conductor. The dimensions of the airline establish the impedance and delay of the airline, and are determined by physical measurements on the airline. The four dimensional features that determine the performance of an airline standard are: 1) length (distance between the mating planes of the outer conductor); 2) the inside diameter of the outer conductor; 3) the outside diameter of the inner conductor; and 4) the difference between the length of the inner and outer conductor (pin depth).
The physical measurements of the airline determine its electrical characteristics and also provide its traceabilty to national standards. Dimensional measurements are made at 20xc2x0 C., and these dimensions are compensated to 23xc2x0 C., where the electrical measurements are made. These compensations are done knowing the thermal coefficient of expansion of the base material of the airlines, usually BeCu.
The electrical calibrations and verifications are done on a microwave network analyzer by connecting various standards to its test ports. Although the ambient temperature is 23xc2x0 C. in the lab during the electrical test, the actual test port temperature is usually higher, due to the heat given off by the internal components in the analyzer which is then conducted through the test ports. Until recently, the microwave network analyzer was only tested for its immunity to ambient temperature fluctuations, and the temperature of the standards themselves was never directly measured.
Recent tests confirmed that the temperature seen by the standards is higher than 23xc2x0 C., and the type of test set, its mounting conditions (and ventilation) all have an effect on the temperature as seen at the standard when it has been connected to the test port. This elevated temperature causes the airlines to expand, which affects the measurement accuracy and stability, especially at high frequencies. The accuracy is affected because the thermal expansion of the airline changes its electrical properties, some of which are derived from the length of the airlines. The stability is affected because of the additional heating due to handling the parts as the connection is being made. Since the parts may be heating up or cooling down during the measurement sequence, the airlines may not have the same dimensions at the beginning of the measurement as they had at the end.
It is an object of the present invention to provide an isothermal port and method for a microwave network analyzer.
The present invention provides an isothermal port and method for maintaining a relatively constant temperature at the test port of a network analyzer. The present invention maintains a relatively constant temperature at the standard itself with a feedback control capability.
In one embodiment, the present invention provides an isothermal port for a microwave network analyzer where the analyzer includes a test port. The present invention includes means for sensing the temperature at the test port; processor means for processing the sensed temperature of the test port over time to generate a temperature control signal; and temperature control means responsive to temperature control signal for maintaining a relatively constant temperature at the test port.